Until now, screenshots with the video filter didn't add subs (unclear
whether that was an oversight or feature). Fix this and make behavior
when taking screenshots with vf_screenshot more consistent with VO
screenshots.
The change in vf_screenshot is needed, because add_subs() checks this
flag to decide whether it's allowed to mutate the image.
This commit has another user visible side effect. When taking a
screenshot each frame (using the "each-frame" mode of the screenshot
command), a normal screenshot command will stop the each-frame mode.
wm4
10a1ae7b72